You can cycle with the betta in it, but that will mean frequent water changes to keep the ammonia levels in the safe zone (0.25ppm or less).

With just one betta in a 30 gallon, the ammonia levels shouldn't rise that quickly so I would say go ahead and add him in there.

However, you mentioned that you also have a pleco. Depending on its size the pleco can have a pretty significant bioload. Even if it is small now it will grow up to 2 feet and so a 30 gallon tank isn't really appropriate for one.